Currently, zeolites, having properties such as molecular-sieve effects due to pores caused by the framework structures of the zeolites, ion exchange ability, catalytic ability, and adsorption ability, have been widely utilized as adsorbents, ion exchange agents, industrial catalysts, and environmental catalysts.
The basic unit of a zeolite structure is the tetrahedral structure of SiO4 or AlO4 (collectively referred to as “TO4 tetrahedron”). An RHO-type zeolite is composed of a body centered cubic arrangement of truncated cuboctahedra or α-cages linked by a double eight-membered ring. RHO-type aluminosilicate zeolites have received attention as separation materials because of exhibiting especially excellent flexibility for cations, gas molecules, water molecules, and the like due to framework structures peculiar to the RHO-type aluminosilicate zeolites (Patent Documents 1 and 2).
As an aluminosilicate zeolite, a zeolite having a practically high SiO2/Al2O3 molar ratio (hereinafter referred to as “SAR”) has been demanded in view of the stability, catalyst performance, and the like of the zeolite. It has been considered that in the case of using the zeolite for a use such as, for example, a catalyst for purifying exhaust emissions, the trivalent element-oxygen-tetravalent element bond (for example, Al—O—Si) of the zeolite is cleaved to separate the trivalent element (for example, Al), which decreases the activity of a metal supported as a catalyst, thereby deteriorating the ability of the catalyst, under the situation of high temperature and the presence of moisture. Further, damage to the structure itself of a zeolite is problematic even if the zeolite is not for a catalyst. It is expected that possible obtainment of a zeolite having a high SAR results in the decreased number of bonds between a trivalent element (for example, Al) and oxygen, and therefore enables stability to be improved and the deterioration of catalytic activity to be prevented. Thus, many zeolites having high SAR have been conventionally synthesized using organic compound structure-directing agents (SDAs). RHO-type aluminosilicate zeolites synthesized without any SDAs have low SAR (about 3) and contain large amounts of impurities (Non Patent Document 1 and Patent Document 3). On the other hand, in 1995, J. Patarin et al. synthesized an RHO-type zeolite having a high SAR of 8.8 using 18-crown-6-ether as an SDA (Non Patent Document 2). Since then, any synthesized RHO-type zeolite having an SAR of more than 8.8 has not been obtained although researchers have examined various synthetic conditions and have attempted synthesis of an RHO-type zeolite having a higher SAR (Non Patent Documents 3 to 6).
Further, Non Patent Documents 7 to 9 propose methods for producing an RHO-type aluminosilicate zeolite having high stability by subjecting a synthesized RHO-type aluminosilicate zeolite to high-temperature steaming treatment to extract some of Al atoms in the framework from the framework, thereby synthesizing the zeolite having a high SAR with respect to the Al atoms present in the framework of the zeolite. However, in the RHO-type aluminosilicate zeolite prepared by the methods, the Al atoms extracted from the framework remain as AlOx seeds in the pores of the zeolite, and therefore, it is impossible to enhance the overall SAR of the zeolite including the inside and outside of the framework. Further, the AlOx seed molecules preclude entrance into and exit from the internal cages and the pores of the zeolite, and it is impossible to sufficiently obtain performances such as ion exchange ability, catalytic ability, and adsorption separation ability for a zeolite.